1. Field of the Invention
The present invention relates to an alignment mark and an alignment mark design method by which wafers are aligned to correspond with masks in an exposure aligning system.
2. Description of the Related Art
Generally, semiconductor devices are fabricated by performing repeated and selective processes such as patterning, etching, diffusing, metal-deposition, etc., to form one or more circuit patterning layers on a wafer. The deposition of circuit patterning layers requires that previously formed circuit patterning layers be aligned accurately.
In the alignment relationship of wafers, the alignment marks formed on exposure field regions EFn or scribe lines SCL of a wafer are illuminated by a light source and reflect diffracted light rays. The diffracted light rays from the alignment marks are detected to generate a photoelectric signal which is used to detect the position of the wafer. The position state of the wafers is checked and the wafers are aligned by an alignment means to meet predetermined reference positions to complete the alignment of the wafers.
In such a conventional technique, the alignment marks shown in FIGS. 1A to 2B are used in a through the lens (TTL) type of field image alignment system in which a red color laser beam (633 nm) is used, and are generally formed of patterns in which a plurality of unit marks are aligned at both sides of the reference position thereof.
The unit mark of the alignment marks shown in FIG. 1 has a width of about 6 to 8.5 and length (l) of about 30 to 40 in a rectangular form. The gap Q between the unit marks is in the range of about 6 to 8.5 to which the width (t) of the unit mark is similar in the aligning direction of the plurality of unit marks. The pitch (P) is in the range of about 12˜17.
If these alignment marks are changed in form in the course of performing various process steps, or covered with process layers including metal layers, there is a problem that both sides of the unit mark are not in a symmetrical form (asymmetry), as shown in FIG. 3. Such an asymmetrical form in the unit mark causes a range of error (d) in the position detection as shown in FIG. 4, which shows a graph illustrating the relationship of detected positions to a signal waveform (contrast).
In addition, the aforementioned unit mark has therein a dishing phenomenon by which its center position gets depressed compared with both end sides of the unit mark during a chemical mechanical polishing CMP process, as shown in FIG. 5 or 6. This phenomenon also causes the range of error (d′) in the position detection of the alignment mark, as shown in FIG. 7, which shows a graph illustrating the relationship of the position detection by contrast.
In an attempt to solve such problems, Korea Patent Application No. 10-2000-47405 discloses changing a conventional rectangular type of unit mark alignment pattern to a matrix type in the alignment marks. However, such a technique cannot be performed on a conventional alignment system. In addition, there is a problem that accuracy in measurement of alignment position is low, and reliability in the position detection is also reduced because the position of the alignment marks is measured by using a predetermined single wavelength of light that does not affect the sensitivity to light, of photoresist.